If you are learning this topic at school, here is a video clip that will help you to understand this topic better. You can replay the video clip as many times as you like until you are really sure of the method.
Click here to view video clip.
Once you have gone through the video clip you can now test yourself and assess how well you understand the topic.
Click here to view test.
Many thanks to emaths.co.uk and BBC bitesize for making the video clip and test pages available for free to all.